Many imaging devices incorporate a prismatic element which is traversed by light that is used to form the image produced by the device. Magnifying binoculars and skin pattern detection devices are examples of such types of devices.
In particular, a prior art skin pattern detection device is represented in FIG. 1. The device, denoted with the reference 100, comprises a prism of transparent material 1, an objective 2, and an image sensor 3. These elements are arranged so that when the device is used, the sensor 3 captures an image formed by the objective 2 from light rays which have passed through the prism 1. The prism 1 generally consists of a homogeneous material, such as polymethyl methacrylate (PMMA) or glass for example. It has an entrance face E which is flat and an exit face S which is also flat or which can be considered to be flat in a first explanation of the operation of the device. The entrance face E and exit face S together form an angle α, it being understood that the faces E and S are not necessary adjacent in the prism 1. In other words, the prism 1 does not necessarily have a single edge between the faces E and S, as there can be an intermediate face between the faces E and S. In this case, the angle α of the prism 1 appears between planes that extend the faces E and S. The objective 2 can be of any type, with three or four lenses for example, and the sensor 3 comprises a photosensitive surface Σ which is formed by a matrix of photodetectors.
The image sensor 3 is arranged so that the objective 2 optically conjugates the entrance face E of the prism 1 with the photosensitive surface Σ of the sensor. To do this, the surface Σ is generally sloped by an angle γ relative to a plane which is perpendicular to an optical axis Δ of the objective 2. In this manner, each point of the surface Σ is the image of a point of the entrance face E. FIG. 1 shows beams of light B1 and C1 forming different image points P1′ and Q1′ on the surface Σ, which are respectively conjugate with two distinct points P1 and Q1 of the entrance face E. In other words, the objective 2 produces on the surface Σ a clear image of a pattern located on the entrance face E.
When a user of the skin pattern detection device of FIG. 1 places the tip of one of his fingers F on the entrance face E, an image of his fingerprint is produced on the surface Σ of the sensor 3 and is captured. The skin surface of the finger F has a pattern of alternating ridges R and grooves G. When the user places his finger F on the face E, the peaks of the ridges R are in contact with the entrance face E and the grooves G are receded relative to this face so that an air gap separates them from it.
The light used to form the image of the fingerprint captured by the sensor 3, usually visible or infrared light, can be introduced by several methods which are also known.
In a first such method, it can be introduced into the finger F through the skin in the vicinity of the fingertip applied to the face E. The light then propagates inside the finger F and exits at the ridges R in contact with the material of the prism 1 on the entrance face E.
In a second light introduction method, represented in FIG. 1, a light source 4 illuminates the face E of the prism 1, through a supplemental face T of the prism 1. The supplemental face T is positioned so that a ray produced by the source 4 and entering the prism 1 by this face T, in the direction of the face E, is reflected inside the prism 1 and exits through the face S in the direction of the objective 2. In addition, the faces E and S of the prism 1 and the optical axis Δ of the objective 2 are oriented so that the internal reflection of the ray on the face E is total when a skin groove G is present at the point of this reflection, and is partial when a skin ridge R is in contact with the face E at the point of reflection. To achieve this, and when the prism 1 is of PMMA, the optical axis Δ of the objective 2 forms an angle β which can be approximately 48° with a direction perpendicular to the face E. For a PMMA/air interface, the critical angle is about 42°. For such a second lighting method, the grooves G appear bright in the image captured by the sensor 3 while the ridges R appear dark. Such a second lighting method corresponds to shadowgraph imaging conditions.
In a third method, the light can be introduced into the prism 1 through yet another face of the prism 1 which is opposite the face E and is denoted W. Preferably, this light can be diffuse. Under these conditions, the grooves G of the fingerprint appear dark in the image captured by the sensor 3, while the ridges R appear light. This third lighting method is a direct lighting method. When there is a supplemental face T of the prismatic element 1, enabling total reflection on the face E, an absorbent screen can be placed opposite the supplemental face T, or the supplemental face T can itself be made to be absorbent.
Lastly, it is also known to give a curved shape to the exit face S of the prism 1, for example a spherical shape, so that this face has additional optical power which combines with that of the objective 2.
Such an imaging device as illustrated in FIG. 1 is limited to the detection of a skin pattern, and is not designed to provide supplemental information. When such supplemental information is desired, an additional imaging device must usually be coupled with the one in the figure to produce an additional image containing the supplemental information. The complexity of the resulting imaging assembly, its bulk, and its cost are then much greater than those of the device in FIG. 1.
Patent WO 96/13742 (the disclosure of which is hereby incorporated by reference) describes, particularly in relation to FIG. 13 of said patent, an imaging device which simultaneously captures an image of a fingerprint of a finger applied to a surface, and of a barcode presented to an entrance face of a prismatic element. Because the face where the finger is applied and the entrance face of the prismatic element form a re-entrant angle, the captured image is not sharp for either the fingerprint or the barcode.